Catalytic Processes for Stereoselective Radical Cyclization Reactions Cyclic structures, including both carbocycles and heterocycles, are common motifs of natural products and synthetic compounds with important biomedical activities. Among different approaches for preparing cyclic molecules, radical cyclization represents one of most powerful approaches for construction of ring structures. With a number of inherent synthetic advantages, radical reactions typically precede at fast reaction rates under mild and neutral conditions in a broad spectrum of solvents and show significantly high functional group tolerance. Furthermore, radical processes have the capability to perform in a cascade fashion, allowing for the rapid construction of complex molecular structures with multiple stereogenic centers. To further enhance the synthetic applications of radical cyclization, new approaches will be needed for achieving high control of their reactivity as well as stereoselectivity, especially enantioselectivity, challenging issues that are intrinsically associated with the free nature of radical chemistry. Guided by the concept of metalloradical catalysis, this proposed research applies a fundamentally new approach for controlling stereoselectivity of both C- and N-centered radical reactions. Cobalt(II) porphyrins [Co(Por)] as stable metalloradicals can enable the activation of diazo reagents and azides to cleanly generate C- and N-centered radicals, respectively, with N2 as the only byproduct in a controlled and catalytic manner. The initially formed C- and N-centered radicals, which remain complexed with [Co(Por)] and are termed as cobalt-carbene and -nitrene radicals, respectively, can undergo common radical reactions such as radical addition to alkenes, but with effective control of reactivity and stereoselectivity by the porphyrin ligand environment. In addition to the radical nature of [Co(Por)], the low bond dissociation energy of Co-C/Co-N bonds plays a key role for the successful turnover of the Co(II)-based catalytic carbene and nitrene transfers, resulting in effective cyclization reactions. Through the support of porphyrin ligands with tunable electronic, steric, and chiral environments, this general concept of Co(II)-based metalloradical catalysis will be applied for the development of various radical cyclization processes for stereoselective construction of both carbocyclic and N-heterocyclic compounds with different ring sizes and varied degrees of molecular complexity. We hope these studies will ultimately lead to the development of cost-effective and environmentally benign radical cyclization processes that can be successfully applied toward the stereoselective synthesis of biologically important natural products and pharmaceutically interesting small molecules.